1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile or a composite machine thereof, in which charging, optical writing, development, transfer, cleaning or the like are sequentially repeated to form toner images on an image carrier and transfer the toner images onto transfer material such as paper, cards or the like. The invention further relates to a process cartridge used in such an electrophotographic image forming apparatus.
2. Discussion of the Background
There are known image forming apparatuses in which, for example, as illustrated in FIG. 10, a process cartridge 1 housing an image carrier 3 and a developing device 4 in a cartridge case 2 is detachably mounted in a body of the apparatuses.
The developing device 4 includes a developing roller 5 and a developer storage portion 6 storing a magnetic toner as a one component developer. The magnetic toner adhered to the developing roller 3 is applied to an electrostatic latent image on the image carrier 5 with the developing roller 5 and thereby the latent image is developed into a toner image. The toner image formed on the image carrier 3 is transferred, by a transfer roller 7, to a transfer material p timely delivered by resist rollers 8.
In the image forming apparatus as described above, as illustrated in FIG. 11, the distance h between the peripheral surface of the developing roller 5 and the upper surface of a bottom portion 2a of the cartridge case 2 is set to be on the order of 1.5 to 2 mm, and the magnetic toner passing therebetween is held by the magnetic force of the developing roller 5. Therefore, generally, the toner in the developer storage portion 6 rarely passes between the peripheral surface of the developing roller 5 and the upper surface of the bottom portion 2a of the cartridge case 2 to adhere to a case portion 2b close to the surface of the image carrier 3 after development of a latent image and before transfer of the developed image.
However, as illustrated in FIG. 12, there exists a problem that a toner t scattered from the surface of the image carrier 3 or the surface of the developing roller 5 is adhered to the case portion 2b and is formed into a lump. The toner lump is removed by some chance to fly out of the cartridge case 2 and thereby contaminates the transfer material p. Further, when the user""s hand touches the case portion 2b for taking out the process cartridge 1, the toner adhered to the case portion 2b contaminates the hands.
Because of the above-described problem, among the known image forming apparatuses, there is an apparatus in which for example, as illustrated in FIG. 13, a conductive member 9 is provided such that the whole surface of the inner surface of the case portion 2b is covered by the conductive member 9, and a voltage of the same polarity as the charging polarity of the magnetic toner is applied to the conductive member 9. The conductive member 9 is provided in parallel with the developing roller 5, for example, as illustrated in FIG. 14. Further, both ends 9a thereof are pulled outside the cartridge case 2 and are superposed on the outer surface of the cartridge case 2, and are electrically connected to a roller shaft 5a of the developing roller 5 through a plate spring-like conductive terminal member m.
However, even when the apparatus has such an arrangement as described above, it is difficult to completely prevent the toner t from adhering to the conductive member 9. Further, there sometimes occurs an inconvenience such as that illustrated in FIG. 15, wherein the toner t is adhered to the case portion 2b round the conductive member 9 to contaminate the transfer material p or hands when the process cartridge 1 is taken out by the hands.
The present invention has been made in view of the above-discussed and other problems and addresses the above-discussed and other problems.
Preferred embodiments of the present invention provide a novel process cartridge and a novel image forming apparatus that eliminate the possibility of contaminating the transfer material and hands by the scattered toner.
According to a preferred embodiment of the present invention, an electrophotographic image forming apparatus using a magnetic toner as a developer includes an image carrier and a case housing a developing roller to develop a latent image on the image carrier with the developer. A magnet is provided to a portion of the case adjacent to the surface of the image carrier after development of the latent image and before transfer of the developed image.
The case portion to which the magnet is provided may be part of a cartridge case of a process cartridge, or may be part of a developing case of a developing device, or may be part of the other components of the apparatus.
In the above-described electrophotographic image forming apparatus, the case portion may be made closer to the image carrier than to the developing roller.
Also, in the above-described electrophotographic image forming apparatus, the magnet may be provided to the case portion through a conductive member and a voltage of the same polarity as that of the developer may be applied to the conductive member. Further, the magnet may be provided so that a range in which the magnet can hold the developer ranges from 0.5 to 3 mm from the tip end of the conductive member on the image carrier side.
Further, in the above described electrophotographic image forming apparatus, when the shortest distance between the developer holding range of the magnet and the image carrier is X and the moving linear velocity of the image carrier is Y, the following relation is established; Xxe2x89xa7Y/100.
Furthermore, in the above-described electrophotographic image forming apparatus, the shortest distance between the developer holding range of the magnet and the image carrier may be not less than 1.0 mm.
According to another preferred embodiment of the present invention, an electrophotographic image forming apparatus using a magnetic toner as a developer includes an image carrier and a case housing a developing roller to develop a latent image on the image carrier with the developer. A conductive member is provided to the developing roller side of a portion of the case adjacent to the surface of the image carrier after development of the latent image and before transfer of the developed image, projecting closer to the image carrier than the case portion, and a voltage of the same polarity as that of the developer is applied to the conductive member.
In the electrophotographic image forming apparatus described immediately above, a non-conductive member may be provided to the case portion side of the conductive member so as to overlap with the conductive member. Further, the overlapped conductive and non-conductive members may be made such that the portion of the overlapped conductive and non-conductive members protruding from the case portion is bent by 1 mm with the bending force being not less than 20 gf/cm when a transfer material comes in contact with the non-conductive member.